The goal of this proposal is to directly examine the relation between adult cortical synaptic plasticity and perceptual learning. In the first (mentored) part of this proposal, I will utilize a prosthetic device for electrical stimulation and recording, and learn to perform behavioral experiments and make recordings in awake animals. In the second (independent) part, I will use whole-cell and extracellular recording in vivo and in slices of primary auditory cortex (A1) to reveal the neural correlates, mechanisms, and network dynamics involved in learning the significance of sensory input. The mammalian auditory system is plastic, maintaining the capacity for structural and functional reorganization all throughout life. Plasticity is an important feature of A1, especially for processing the behavioral significance of sensory signals such as speech, music and other forms of auditory communication. Mechanisms of cortical plasticity are disrupted in learning impairments and language disorders; conversely, engaging these mechanisms by training programs and prosthetic devices will help repair damaged brains in pathological conditions. Thus understanding the rules, cellular mechanisms, and functional relevance of cortical plasticity is essential for language learning and mental health. Patterns of sensory input control the organization and plasticity of cortical receptive fields, depending on activation of excitatory and inhibitory circuits. Subcortical neuromodulator systems are also necessary for cortical plasticity, reflecting the importance of attention and behavioral context for learning. Behaviorallyengaged neuromodulators have a wide range of effects in cortex and throughout the rest of the brain, raising the questions of how attention and neuromodulation govern cortical networks to induce modification of these circuits, and how such changes in turn affect auditory perception and behavior. In this proposal, I will perform behavioral and electrophysiological experiments to determine the mechanisms and functional significance of A1 plasticity in adult rats. I will thoroughly examine A1 plasticity at the synaptic, network, and behavioral levels to provide a unified description of the neural correlates of perceptual learning.